Lubricating oil for rolling bearing in high-speed rotating equipment, and bearing lubricated with the same lubricating oil

ABSTRACT

A lubricating oil for a rolling bearing in a high-speed rotating equipment comprises a nondrying vegetable oil. A rolling bearing for a high-speed rotating equipment is provided with a rolling bearing element constructed of an outer ring, an inner ring, rolling elements and a retainer to rotatably support at least a rotary shaft thereon. The retainer is made of a non-porous metal or synthetic resin and is lubricated with the lubricating oil.

This is a Divisional Application of application Ser. No. 08/940,426,filed Sep. 30, 1997 allowed.

BACKGROUND OF THE INVENTION

a) Field of the Invention

This invention relates to a high-performance lubricating oil applied toa rolling bearing as a principal element in a high-speed rotatingequipment employed in a field where high speed and high safety arerequired, such as a medical, food-processing or aerospace equipment, andalso to a rolling bearing lubricated with the lubricating oil.

More specifically, the present invention is concerned with a novelhigh-performance lubricating oil applied to a high-speed rotatingequipment employed in a medical, food or like field, such as ahigh-speed cutter (typically, an air turbine hand piece), especially toa rolling bearing as a principal element of the equipment, and also witha rolling bearing lubricated with the lubricating oil and suited for usein a high-speed rotating equipment.

In particular, the present invention pertains to a novelhigh-performance lubricating oil having excellent biosafety (i.e.,reduced harm to the body) and environmental conservation (i.e., safetyto the environment), enabling to achieve a durable lubrication systemand applicable to a rolling bearing element in a high-speed rotatingequipment such as a high-speed cutter (typically, an air turbine handpiece), and also to a rolling bearing lubricated with the lubricatingoil and suited for use in a high-speed rotating equipment.

b) Description of the Related Art

A high-speed rotating equipment, for example, a high-speed cutter isgenerally constructed of a rotary shaft for fixedly holding variouscutting tools thereon, a drive unit for rotating the rotary shaft, and abearing unit rotatably supporting the rotary shaft.

As a high-speed cutter of this type, an odontotherapeutic high-speedcutter (i.e., air turbine hand piece) can be mentioned by way ofexample.

Bearing units for the above-mentioned odontotherapeutic high-speedcutters (i.e., air turbine hand pieces) are known to include thosehaving a ball bearing system making use of balls (rolling elements) andthose equipped with a (non-contact) air bearing system making use of anair bearing.

Paying attention, for example, to the bearing mechanisms of dental airturbine hand pieces, two types of air turbine hand pieces are known, onebeing of the ball bearing turbine type and the other of the air bearingturbine type.

The former type, i.e., the ball bearing turbine type can be consideredto be a high-speed rotation type for approximately 200,000 to 400,000rpm, while the latter type, that is, the ball bearing turbine type canbe considered to be a super high-speed rotation type for approximately300,000 to 500,000 rpm.

It is however to be noted that the above-described revolution speeds ofthe ball bearing turbine type and the air bearing turbine type aregeneral values. For example, the dental air turbine hand piece alreadyproposed by the present inventors (U.S. Pat. No. 5,562,446) is of theball bearing turbine type. Nonetheless, it has high performance so thatsuper high-speed rotation can be achieved.

To facilitate the understanding of the conventional art and thisinvention, a description will now be made about the construction of anequipment to which the lubricating oil according to the presentinvention is applied, namely, of a dental high-speed cutter (i.e., adental air turbine hand piece).

FIG. 1 to FIG. 2 show the construction of the dental air turbine handpiece, in which FIG. 1 is a perspective view illustrating the overallconstruction and FIG. 2 is a cross-sectional view illustrating theinternal construction of a head portion and a neck portion inparticular.

As is depicted in FIG. 1, the dental air turbine hand piece designatedgenerally by letter A is composed of a head portion H, which carries acutting tool B(5) fixedly held on a rotor shaft (drive shaft) of an airturbine, and a grip portion G.

A neck portion N of the grip portion G is connected to the head H, andis internally provided with means for supplying compressed air to theair turbine arranged within the head portion H and also for dischargingcompressed air from the air turbine.

FIG. 2 illustrates the internal construction of the head portion H andthe neck portion N of the dental air turbine hand piece A.

As is illustrated in the figure, in the head portion H, a turbine rotorshaft 3 with turbine blades 2 disposed at a peripheral edge portionthereof is arranged within a chamber 11 of a head 1, and the turbinerotor shaft 3 is rotatably supported within the head 1 by way of abearing unit 4.

The head 1 is composed of a head main part 12 and a cap part 13. Withinthe head main part 12, the bearing unit 4 is arranged to rotatablysupport the turbine rotor shaft 3. To perform treatment, the cuttingtool 5 is fixedly held in a bore which is formed through the turbinerotor shaft 3 along a central axis thereof. Incidentally, the cuttingtool 5 is provided on a peripheral side wall thereof with a chuck 51 forholding the cutting tool 5 in place within the bore.

The bearing unit 4 is of the ball bearing type and is constructed of aninner ring 41, an outer ring 42, rolling elements 43 and a retainer 44.The bearing unit 4 may be provided on an outer periphery or side wallthereof with O-rings for providing the bearing unit with self-centeringfunction and/or with known wave washers for enhancing the rigidity ofthe shaft.

A main part 6 of the neck portion N is provided with an air supplypassage 7 and an air inlet 71 for supplying compressed air to theturbine blades 2 arranged within the chamber 11 and also with airdischarge passages 8,9 and air outlets 81,91 for discharging compressedair from the chamber 11.

In the above-described internal construction of the dental air turbinehand piece A as illustrated in FIG. 2, the means for supplying anddischarging compressed air is the one already proposed by the presentinventors (U.S. Pat. No. 5,562,446) and is of a new construction totallyunseen in the conventional art.

Accordingly, FIG. 2 contains other reference symbols in addition tothose referred to in the above to describe the individual elements(members). Although a description of these additional symbols is omittedherein, the construction of the conventional dental air turbine handpiece can be easily understood on the basis of FIG. 2.

The dental air turbine hand piece A--which is equipped with the airsupply and discharge means shown in FIG. 2 and already proposed by thepresent inventors--belongs to the category of conventional hand pieceswith a rolling bearing built therein, but makes it possible to obtainrotation of an extremely high speed and hence a large torque.

In the above-described dental air turbine hand piece of the ball bearingtype, its bearing unit is in the form of a miniature bearing unit. Sincethe turbine rotor shaft rotates at a high speed of approximately 200,000to 400,000 revolutions per minute, the temperature inside the bearingunit becomes high and further, the bearing unit is exposed to a largestress. For a lubricating oil to be applied to a bearing which is usedunder the above-described severe conditions, it is therefore extremelyimportant to control its quality and properties.

Further, the above-described dental air turbine hand piece of the ballbearing type is used in the oral cavity. Accordingly, the dental airturbine hand piece is used by spraying or dropping a lubricating oil tothe bearing unit, in other words, is used under an environment ofminimum lubrication, and is subjected to high-pressure andhigh-temperature treatment for sterilization and disinfection (which isalso called "autoclaving", which is applied under the followingconditions: steam pressure, 2.4 kg/cm² ; temperature, 135° C.; time, 5minutes).

The lubricating oil for use in the above bearing unit is thereforerequired to have properties sufficient to withstand such severe useconditions, such as oxidation resistance.

As lubricating oils for rolling bearing units in high-speed rotatingequipments, a variety of lubricating oils have been used or proposed todate.

For example, it is widely practiced to supply a lubricating oil byspraying it with flon or LPG. As lubricating oils, those making use ofrefined mineral oils, such as paraffin, as base oils are known.

The above-described lubricating oils are typically of the petroleum baseand are prepared by fractionating and refining petroleum into variousfractions and incorporating additives, such as an oxidation inhibitor,therein as needed.

As base oil components of the above-described lubricating oils, thereare known synthetic oils such as glycol esters and low molecular weightpolyolefins in addition to natural mineral oils. Furthermore, edibleoils such as animal oils and vegetable oils are also known to be in useas lubrication oils for precision machines, machine tools, ship enginesand the like. In general, however, these edible oils are used by beingadded in proportions of from 10 to 20 wt. % to lubricating oil of themineral oil base. Incidentally, these edible oils have a problem inoxidation resistance. It is therefore the common practice to use suchedible oils in combination with one or more of various oxidationinhibitors.

In view of the above-described severe use conditions for dental airturbine hand pieces, dental air turbine hand pieces equipped withretainers impregnated with fluorinated oils, which have excellent heatresistance, permit sterilization and disinfection (autoclaving) and havesuperb lubricity, have been proposed recently in Japanese PatentPublication (Kokoku) No. HEI 5-43884 and Japanese Utility ModelApplication Laid-Open (Kokai) No. HEI 7-10553.

Incidentally, the retainers are in the form of porous members obtainedby sintering powder of a polyimide resin.

The fluorinated oils have properties such that they are inactive, areexcellent in heat resistance, chemical resistance and solvent resistanceand, even when exposed to high temperatures, do not form a soliddeterioration material. It is therefore possible to consider that theabove-described proposed dental air turbine hand pieces have used theseproperties for the retainers.

Further, Japanese Patent Application Laid-Open (Kokai) No. HEI 6-165790discloses an embodiment of impregnating with a lubricating oil a snapretainer of a ball bearing in a dental air turbine hand piece to bedescribed briefly hereinafter, although its direct subject matter is thesnap retainer itself.

Specifically, in the dental air turbine hand piece disclosed in JapanesePatent Application Laid-Open (Kokai) No. HEI 6-165790 referred to in theabove, the retainer of the ball bearing is characterized in that:

(i) the retainer is a snap retainer, in which a ball-holding pocket isformed on one side of a synthetic resin cylinder having a fabric-madefiber layer therein, and chamfered portions are formed on opening-sideend edges of the pocket; and

(ii) the fiber layer of the retainer is impregnated with a lubricatingoil.

Incidentally, the snap retainer of the ball bearing disclosed inJapanese Patent Application Laid-Open (Kokai) No. HEI 6-165790 is toimprove the balance of rotation by the feature (i), thereby making itpossible to protect the retainer from wearing and also to avoid anincrease in rotating torque, both of which would otherwise occur due tocontact between the retainer and the outer ring.

However, the specific composition of the lubricating oil impregnated inthe retainer is not clear in Japanese Patent Application Laid-Open(Kokai) No. HEI 6-165790. As a matter of fact, Japanese PatentApplication Laid-Open (Kokai) No. HEI 6-165790 does not disclosewhatsoever the specific composition of the edible oil, although itdescribes an embodiment of charging an edible oil into a housing in thedescription of the conventional art. It is the present inventors' beliefthat the lubricating oil or edible oil disclosed in Japanese PatentApplication Laid-Open (Kokai) No. HEI 6-165790 is still no better thanthe level of the conventionally-proposed lubrication oils in view of thetechnical level in the present field of art.

Further, Japanese Patent Application Laid-Open (Kokai) No. HEI 6-212179discloses mixing of ceramic powder in a lubricating oil to reduce thereplenishing frequency of a lubricating oil to a bearing of a dental airturbine. This is a proposal based on a finding that advance mixing ofceramic powder in a lubricating oil can improve the life time of thelubricating oil.

The above-described various lubricating oils, which have been proposedto date, are still accompanied by various drawbacks which should beimproved in order to permit their application in high-speed rotatingequipments such as odontotherapeutic air turbine hand pieces havingrolling bearings.

For example, the above-described lubricating oils of the mineral oilbase, such as of the liquid paraffin base, or of the synthetic oil baseor the lubricating oils obtained by adding edible oils to them stillinvolve matters to be improved from the viewpoint of biosafety andenvironmental conservation.

In addition, the fluorinated oils proposed in the above-describedJapanese Patent Publication (Kokoku) No. HEI 5-43884 and JapaneseUtility Model Application Laid-Open (Kokai) No. HEI 7-10553, such asperfluoropolyether (PFPE) and perfluoropolyalkyl ether (PFAE), areexcellent in heat resistance, chemical resistance and solvent resistanceand, even when exposed to high temperatures, form no solid deteriorationmaterial. They are accordingly preferred as lubricating oils forhigh-speed rotating equipments. They however still involve matters to beimproved from the viewpoint of environmental destruction and biosafety.

Furthermore, the above-described Japanese Patent Application Laid-Open(Kokai) No. HEI 6-165790 discloses impregnation of a retainer of arolling bearing, which is formed of a shaped member made of a phenolresin and having a fiber layer, with a lubricating oil, and suggests useof an edible oil as the above-described lubricating oil. As will bedescried in detail subsequently herein, a great majority of generaledible oils are drying oils and, when they become dry, they are formedinto resinous solids. They are accordingly not suited as durablelubricating oils for bearings.

As has been described above, most of general edible oils are dry oilsand are susceptible to oxidation. It is therefore the common practice touse them by adding a synthetic oxidation inhibitor therein. In anedible-oil-base lubricating oil composed primarily of theabove-described drying oil and added with an oxidation inhibitor, it isnecessary to keep in mind the potential risk that a reaction product tobe formed between the oxidation inhibitor and a metal dissolved out froma bearing system may be a material harmful for the body.

According to the above-described Japanese Patent Application Laid-Open(Kokai) No. HEI 6-212179, ceramic powder is mixed in a lubricating oilto improve the life time of the lubricating oil so that the replenishingfrequency of the lubricating oil to a lubrication system can be reduced.In an odontotherapeutic cutter of the high-speed rotation type (airturbine hand piece), however, the race (retainer) and balls of thebearing are ground by the ceramic powder so that a metal harmful for thebody may be dissolved out, or a serious damage may be given to thebearing mechanism when the lubricating oil becomes scarce under theenvironment of minimum lubrication.

As has been described above, the conventional lubricating oils forrolling bearings, said oils being for use in high-speed rotatingequipments, for example, odontotherapeutic high-sped cutters (airturbine hand pieces), are still accompanied by drawbacks when evaluatedin view of the following properties:

(i) biosafety (reduced harm to the body),

(ii) environmental conservation (safety),

(iii) heat resistance (the possibility of sterilization and disinfectionby autoclaving), and

(iv) durable lubrication system.

SUMMARY OF THE INVENTION

The present invention has been completed in view of the above-describedproblems of the conventional art.

Incidentally, the direct motive for the present invention resided in thelack of a lubricating oil of excellent properties for thehigh-performance and high-speed rotating, dental air turbine hand pieceof the ball bearing turbine type previously proposed by the presentinventors (U.S. Pat. No. 5,562,446).

In the present invention, there are proposed a novel lubricating oilhaving excellent properties in connection with the above-describedevaluation characteristics such as biosafety and a rolling bearing for ahigh-speed rotating equipment, said rolling bearing having beenlubricated with the novel lubricating oil having above-mentionedexcellent properties.

In particular, the present invention provides a novel lubricating oilcapable of meeting requirements such that in a high-speed rotatingequipment such as an odontotherapeutic high-speed cutter (air turbinehand piece of the ball bearing type), it is usable under large torquesproduced by high-speed rotation, it has excellent heat resistance, inother words, it can permit sterilization and disinfection by autoclavingunder high temperature and high pressure, and it can be used stably overa long period of time; and also a rolling bearing for a high-speedrotating equipment, said bearing having been lubricated with theabove-described lubricating oil.

In other words, the present invention provides a novel lubricating oilfor a rolling bearing as a principal element in a high-speed rotatingequipment of the ball bearing type operated at 200,000 rpm or higher oreven at 300,000 rpm or higher such as an odontotherapeutic high-speedcutter, said lubricating oil being capable of establishing a durablelubrication system without needing a reduction in revolution speed inthe high-speed rotating equipment; and a rolling bearing lubricated withthe lubricating oil.

Describing the present invention in brief, the present inventionrelates, in a first aspect thereof, to a lubricating oil for a rollingbearing in a high-speed rotating equipment, wherein said lubricating oilcomprises a nondrying vegetable oil.

The present invention also relates, in a second aspect thereof, to arolling bearing for a high-speed rotating equipment, wherein the rollingbearing has been lubricated with the above-described nondrying vegetablelubricating oil.

The lubricating oil according to the present invention, which iscomposed primarily of the nondrying vegetable oil and is applicable tothe rolling bearing in the high-speed rotating equipment such as theodontotherapeutic high-speed cutter (air turbine hand piece) hasexcellent advantages as will be described next.

(i) The lubricating oil according to the present invention, whichcomprises the nondrying vegetable oil, is excellent in variousproperties, i.e., in biosafety, environmental conservation, heatresistance (autoclaving resistance) and bearing durability.

Further, it is also excellent in economy because its cost is lower thanthose of conventionally-proposed lubricating oils of the fluorinated oilbase.

(ii) Vegetable oils other than nondrying vegetable oils, such assemidrying oils and drying oils and paraffin oil (liquid paraffin), areobserved to undergo considerable color changes when tested, namely, leftover at 135° C. for 175 hours in air. In other words, these oils ofvarious varieties are inferior in oxidation resistance.

In contrast, nondrying vegetable oils useful in the present invention,such as olive oil and arachis oil, do not show color changes in theabove-described test, and are excellent in oxidation resistance.

(iii) The lubricating oil according to the present invention, which iscomposed primarily of a nondrying vegetable oil and is applicable to arolling bearing in a high-speed rotating equipment such as a dental airturbine hand piece, has a high biodegradation speed and, even whenevaluated based on the Interim Water Quality Standards for Effluents (anordinance of the Office of the Prime Minister), is more preferable thanconventional mineral oils. Incidentally, according to theabove-mentioned Interim Water Quality Standards for Effluents, thepermissible limit for effluents is 30 mg/liter in the case of vegetableoils as opposed to 5 mg/liter in the case of conventional mineral oils.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a dental air turbine hand piece to whicha lubricating oil according to the present invention, said lubricatingoil comprising a nondrying vegetable oil, is applied;

FIG. 2 is a cross-sectional view of a head portion and a neck portion ofthe dental air turbine hand piece of FIG. 1;

FIG. 3 is a perspective view of a retainer shown in FIG. 2;

FIG. 4 is a cross-sectional view similar to FIG. 2, but illustratesanother dental air turbine hand piece to which the lubricating oilaccording to the present invention, said lubricating oil comprising thenondrying vegetable oil, is applied; and

FIG. 5 is a cross-sectional view of a retainer depicted in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The technical features and embodiments of the present invention willhereinafter be described in detail.

As has been described above, the characteristic feature of the firstaspect of the present invention resides in the novel constitution of thelubricating oil applied to the rolling bearing in the high-speedrotating equipment [for example, an odontotherapeutic high-speed cutter(air turbine hand piece)] having as an essential element the rollingbearing rotatably supporting the rotary shaft thereon.

Specifically, the characteristic feature of the first aspect of thepresent invention resides in that, in a high-speed rotating equipment[for example, a high-speed cutter (air turbine hand piece)] having arolling bearing element composed an outer ring, an inner ring, rollingelements (ball bearings) and a retainer made of a metal or aheat-resistant synthetic resin and rotatably supporting thereon a rotaryshaft with air turbine blades fixed thereon, a lubricating oil appliedto the rolling bearing element is composed of a nondrying oil out ofvegetable oils excellent in biosafety and environmental conservation andsuperb in heat resistance (so that sterilization and disinfection byautoclaving is feasible) and durability in place of a conventionallubricating oil of the mineral oil base or of the synthetic oil base.

First, a description will be made in detail about the characteristicfeature of the first aspect of the present invention that thelubricating oil is composed of a vegetable oil, especially a nondryingvegetable oil.

Roughly dividing, vegetable oils can be classified into the followingthree types:

(i) Nondrying oils

The term "nondrying oil" means an oil which does not form any film-likematter (resinous solid) even when dried (oxidized) in the form of a thinlayer in air.

A nondrying oil of this type contains unsaturated fatty acids, each ofwhich contains two or more double bonds per molecule (hereinafter called"polyunsaturated fatty acids"), only in small amounts and is primarilycomposed of the glyceride (glycerol ester) of oleic acid (which containsone double bond per molecule), and its iodine value (a scale indicatingthe degree of unsaturation of an oil) is 100 or smaller.

Representative examples of nondrying oils of this type can include oliveoil, arachis oil and oleysol oil.

(ii) Semidrying oils

The term "semidrying oil" means an oil which shows intermediateproperties between a nondrying oil and a drying oil to be describednext. Its iodine value is from 100 to 130.

Representative examples of semidrying oils of this type can includerapeseed oil, sesame oil and cotton seed oil.

(iii) Drying oils

The term "drying oil" means an oil which forms a film (resinous solid)when dried (oxidized) in the form of a thin layer in air. A drying oilof this type is composed of glycerides of fatty acids having highdegrees of unsaturation (for example, linoleic acid contains two doublebonds and linolenic contains three double bonds). These glyceridesabsorb oxygen in air and undergo oxidative polymerization, whereby afilm-like matter is easily formed. Incidentally, the iodine value ofsuch a drying oil is 130 or greater.

Typical examples of drying oils of this type can include linseed oil andtung oil.

Among the above-described various vegetable oils, nondrying oils areoils and fats (glycerol esters of fatty acids) each of which does notform a film-like matter (resinous solid) even when dried (oxidized) inthe form of a thin layer. As they are excellent in heat resistance (sothat sterilization and disinfection by autoclaving is feasible) anddurability, they are suitable as lubricating oils for rolling bearingsin high-speed rotating equipments such as dental air turbine handpieces.

As a lubricating oil for a rolling bearing in a high-speed rotatingequipment such as a dental air turbine hand piece, the present inventionadopts a nondrying oil out of vegetable oils.

A description will hereinafter be made in detail about olive oil, arepresentative example of the above-described nondrying vegetable oils.

Olive oil is an oil (glycerol esters) available from drupes of OleaEuropaea. Roughly driving, its components can be classified into thefollowing three types:

(i) unsaturated resin acids;

(ii) saturated resin acids; and

(iii) various trace components.

The unsaturated resin acids in olive oil are generally composed ofmono-unsaturated and di- and higher-unsaturated (poly-unsaturated) resinacids.

The kinds and contents of the unsaturated resin acids in olive oil willbe shown below:

1) Oleic acid (mono-unsaturated) . . . 56.0-83.0%

CH₃ (CH₂)₇ CH═CH(CH₂)₇ COOH

2) Linoleic acid (poly-unsaturated) . . . 3.5-20.0%

CH₃ (CH₂)₄ CH═CHCH₂ CH═CH(CH₂)₇ COOH

3) Palmitoleic acid (mono-unsaturated) . . . 0.3-3.5%

CH₃ (CH₂)₅ CH═CH(CH₂)₇ COOH

4) Linolenic acid (poly-unsaturated) . . . 0.0-1.5%

CH₃ CH₂ CH═CHCH₂ CH═CHCH₂ CH═CH(CH₂)₇ COOH

5) Gadoleic acid (mono-unsaturated) . . . 0.0-0.05%

CH₃ (CH₂)₉ CH═CH(CH₂)₇ COOH

As has been described above, olive oil abundantly contains oleic acidwhich is a mono-unsaturated fatty acid. Olive oil also containspoly-unsaturated fatty acids, such as linoleic acid, in small amounts.

As has also been described above, a poly-unsaturated fatty acid issusceptible to oxidation. Nonetheless, olive oil has excellent oxidationresistance as a whole because, as will be described subsequently herein,olive oil contains tocopherols (vitamin E) as trace components andpoly-unsaturated fatty acids such as linolenic acid are protected fromoxidative deterioration owing to the anti-oxidation action of thetocopherols (vitamin E).

A description will next be made about saturated fatty acid components inolive oil.

The kinds and contents of saturated fatty acids in olive oil will beshown below:

1) Palmitic acid CH₃ (CH₂)₁₄ COOH . . . 7.5-20.0%

2) Stearic acid CH₃ (CH₂)₁₆ COOH . . . 0.5-3.5%

3) Myristic acid CH₃ (CH₂)₁₂ COOH . . . 0.0-0.05%

4) Arachidic acid CH₃ (CH₂)₁₈ COOH . . . 0.0-0.05%

5) Behenic acid CH₃ (CH₂)₂₀ COOH . . . 0.0-0.05%

6) Lignoceric acid CH₃ (CH₂)₂₂ COOH . . . 0.0-0.05%

As is appreciated from the foregoing, olive oil can be considered tohave low contents of saturated fatty acids which causehypercholesterolemia.

Next, a description will be made of various trace components in oliveoil.

The kinds of various trace components in olive oil will hereinafter bedescribed along with their properties and functions.

(1) Unsaponifiable materials:

(a) Sterols

(b) Hydrocarbons

Squalene

Aromatic hydrocarbons (which impart inherent sensory characteristics,namely, aroma and flavor)

(c) Tocopherols (oxidation-preventing function)

α-tocopherol (vitamin E) (prevention of blackening and polymerization)

β,γ,δ-tocopherols (prevention of rancidity which would otherwise becaused by the existence of one or more heavy metals)

(d) Triterpene alcohols

Cycloalterenol

Erythrodiol

(e) Fat-soluble vitamins

Vitamins A, D (anti-oxidation effects)

(2) Phospholipids, chlorophyll and derivatives

(a) Phospholipids

(b) Chlorophyll (anti-oxidation effect)

(3) Phenolic compounds

(a) Phenolic compounds (anti-oxidation effects)

(b) Polyphenols (anti-oxidation effects)

As is appreciated from the foregoing, olive oil has higher contents ofvarious trace components, which act against oxidation of oils and fats,than other non-drying oils and drying oils, and can therefore provide alubricating oil excellent in heat resistance (so that sterilization anddisinfection by autoclaving is feasible) and durability.

A description will next be made about other nondrying vegetable oilswhich can also make up the lubricating oil according to the presentinvention for a bearing in a high-speed rotating equipment such as adental air turbine hand piece.

(i) As a nondrying oil other than the above-described olive oil, thereis arachis oil.

Arachis oil is found at a content of 40-50% in seeds of Arachishypogaea, and is derived from its seeds by pressing.

(ii) As a nondrying oil other than the above-described olive oil, thereis oleysol oil.

Oleysol oil is available from a mutant species of sunflower, which has ahigh content of linoleic acid (poly-unsaturated) which is not anondrying oil. Owing to the efforts of agricultural chemical engineersto date, it has been succeeded in growing a sunflower of mutant specieswhich abundantly contains oleic acid (mono-unsaturated fatty acid). Fromthis mutant species, oil named "Oleysol" is produced. Oleysol oil is anondrying oil similar to the above-described olive oil.

Differences between certain nondrying vegetable oils, which can make uplubricating oils according to the present invention for high-speedrotating equipments such as dental air turbine hand pieces, and somesemidrying vegetable oils and other edible oils are shown below inTable 1. In Table 1, olive oil, arachis oil and oleysol oil arenondrying vegetable oils useful for the practice of the presentinvention, while the remaining oils indicate semidrying vegetable oilsand drying oils as controls.

In Table 1, the notes have the following meanings:

(1) composed primarily of oleic acid, and containing palmitoleic acid,

(2) linoleic acid,

(3) linolenic acid, and

(4) composed of palmitic acid, stearic acid, lauric acid and myristicacid.

In Table 1, asterisk (*) indicates control vegetable oils.

                  TABLE 1                                                         ______________________________________                                        Comparison between Nondrying Vegetable Oils and Other Edible Oils                                            Ratio of                                       Average constituent fatty acids (%)                                                                          Vitamin E/                                     Unsaturated fatty Saturated        poly-                                      acids             fatty    Vitamin unsaturated                                Mono-       Di-   Tri-    acids  E     fatty acids                            (1)         (2)   (3)     (4)    (mg/kg)                                                                             (mg/g)                                 ______________________________________                                        Olive oil                                                                             70      12    0.5-1.5                                                                             15     150   1.11                                 Arachis oil                                                                           60      25    --    15     150   0.60                                 Oleysol oil                                                                           80      10          10                                                rapeseed oil*                                                                         60      22    10     8     150   0.47                                 Sunflower                                                                             20      70    10    10     250   0.315                                oil*                                                                          Soybean oil*                                                                          23      55     7    15     175   0.28                                 Corn oil*                                                                             26      60     2    12     200   0.32                                 Coconut oil*                                                                           8       2    <1    90                                                (solid fat)                                                                   ______________________________________                                    

From Table 1, the following tendencies are apparent.

(i) A nondrying vegetable oil contains oxidation-resistantmono-unsaturated fatty acids in a large total amount.

(ii) A nondrying vegetable oil contains oxidation-susceptible di- totri-unsaturated fatty acids, namely, poly-unsaturated fatty acids in asmall total amount.

(iii) A nondrying vegetable oil contains tocopherols (vitamin E and thelike) having anti-oxidation effects at a high ratio relative topoly-unsaturated fatty acids.

In nondrying vegetable oils (olive oil, arachis oil, oleysol oil, andthe like) which can make up lubricating oils of this invention forrolling bearings in high-speed rotating equipments such as dental airturbine hand pieces, lubrication characteristics becomes better as thetotal content of free fatty acids becomes lower.

These tendencies were found in the course of the present inventors'investigation toward improvements in the lubricating characteristics ofnondrying vegetable oils and, as will be explained subsequently herein,are supported by substantiating data.

Concerning the above-mentioned fatty acids (which may hereinafter becalled "free fatty acids") liberated into a nondrying vegetable oil, adescription will hereinafter be made.

In general, an oil or fat (a fat such as beef tallow, lard or butter; ora fatty oil such as rapeseed oil, tung oil or linseed oil) is composedof glycerol esters of higher fatty acids.

Namely, in a nondrying vegetable oil useful in the practice of thepresent invention, various fatty acids (saturated and unsaturated) existas esters represented by the following formula:

    3 molecules of fatty acid

    +1 molecule of glycerol

    →1 molecule of triglyceride (ester)                 (1)

However, the nondrying vegetable oil also contains various fatty acids(free fatty acids) which are not combined with glycerol (CH₂OH--CHOH--CH₂ OH). Expressing the total content of the above-mentionedfree fatty acids in terms of free acid value, the lower this value, thelower the acidity and the more shifted toward a higher viscosity sidethe viscosity. A non-drying vegetable oil having a lower free acid valueis therefore excellent in durability as a lubricating oil for a rollingbearing.

Based on the above-described free acid value, olive oil is classified inquality as shown below in Table 2. As will be apparent from Table 2,olive oil of higher quality has a lower free acid value and, as will bedescribed subsequently herein, shows better lubricating characteristics(see Table 3).

To lower the free acid value of a nondrying vegetable oil such as oliveoil, a method to be described next can be adopted by way of example.Namely, when olive oil is heated subsequent to addition of a 5-10%aqueous solution of sodium hydroxide, the olive oil is saponified toform glycerol and the sodium salts of fatty acids. The resultingglycerol esterifies free fatty acids. Subsequent removal of oils andfats by centrifugation can provide olive oil having a low free acidvalue.

In Table 2, the names of various grades of olive oil are trade names ofolive oils produced by Golden Eagle Olive Products, U.S.A.

                  TABLE 2                                                         ______________________________________                                        Free Acid Values of Olive Oil                                                 Grade of olive oil                                                                          Free acid value (wt. %)                                         ______________________________________                                        Refined oil   0.1                                                             Extra virgin oil                                                                            1.1                                                             Virgin oil    3.2                                                             ______________________________________                                    

As an additive to the nondrying vegetable oil (olive oil, arachis oil,oleysol oil or the like) as a principal component in the lubricating oilof this invention for the rolling bearing in the high-speed rotatingequipment such as the dental air turbine hand piece, particles of anoil-absorbing synthetic resin can impart excellent characteristics to alubrication system under an environment of high-speed rotation.

The present inventors have found that particles of an oil-absorbingsynthetic resin (oil-absorbing crosslinked polymer) such as acrosslinked polymer of the acrylate ester type are extremely effectivefor improving the retention of the lubricating oil for the rollingbearing without impairing the properties of the nondrying vegetable oil.

A description will hereinafter be made about the oil-absorbingcrosslinked polymer which is preferred for improving the properties ofthe nondrying vegetable oil useful in the practice of the presentinvention.

Incidentally, the oil-absorbing crosslinked polymer itself is known inthe present field of art from Japanese Patent Application Laid-Open(Kokai) No. HEI 5-337367 or Japanese Patent Publication (Kokoku) No. HEI3-143996. Further, needless to say, it has been found for the first timeby the present inventors that the above-mentioned oil-absorbingcrosslinked polymer imparts excellent characteristics to a rollingbearing in a high-speed rotation system.

The nondrying vegetable oil as the principal component in thelubricating oil has a solubility parameter (SP value) of from 6 to 9. Itis therefore preferred for the above-described oil-absorbing crosslinkedpolymer to have a similar SP value from the viewpoint of compatibility.Namely, it is preferred to add an oil-absorbing crosslinked polymer of 9or smaller in SP value to the lubricating oil according to the presentinvention, said lubricating oil containing the nondrying vegetable oilas a principal component.

The above-described oil-absorbing crosslinked polymer can be producedgenerally by copolymerization of:

(i) 90 to 99.9 wt. % of a monomer (A) containing one polymerizableunsaturated group per molecule and capable of producing a polymer havingan SP value of 9 or smaller, and

(ii) 0.1 to 10 wt. % of a crosslinking monomer (B) containing at leasttwo polymerizable unsaturated group per molecule.

The monomer (A) comprises at least one polymerizable unsaturated monomercontaining:

(1) at least one C₂ -C₃₀ aliphatic hydrocarbon group; and selected fromthe group consisting of:

(2) alkyl (meth)acrylate, alkylaryl (meth)acrylate,alkyl(meth)acrylamide, alkylaryl (meth)acrylamide, alkylaryl(meth)acrylamdie, fatty acid vinyl esters, alkylstyrenes, and α-olefins.

Illustrative of the crosslinking monomer (B) are ethylene glycoldi(meth)acrylate, diethylene glycol di(meth)arylagte, polyethyleneglycol di(meth)acrylate, propyleneglycol di(meth)acrylate, polypropyleneglycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, neopentylglycol di(meth)acrylate, N,N'-methylene bisacrylamide,N,N'-propylenebisacrylamide, glycerol tri(meth)acrylate,trimethylolpropane tri(meth)acrylate, and divinylbenzene.

The above-described oil-absorbing crosslinking polymer can also beproduced by using a monomer containing two polymerizable unsaturatedgroups per molecule, specifically a diene monomer.

Illustrative examples of oil-absorbing crosslinked polymers making useof diene monomers of this type can include polymers of butadiene,isoprene, cyclopentadiene and 1,3-pentadiene, and their hydrogenationproducts; and copolymers of the above-described dienes with otherpolymerizable monomers such as styrene and α-olefins such as butylene,and their hydrogenation products.

Incidentally, the above-described crosslinking monomers can be used asthe polymerizable monomers.

Further, the oil-absorbing crosslinked polymer may be composed of acrosslinked copolymer of ethylene with another olefin. Examples of theanother olefin copolymerize with ethylene can include propylene,butylene and pentene. Further, as crosslinking monomers, those mentionedabove can be used.

The above-described oil-absorbing crosslinked polymer is in the form ofparticles the average particle size of which is form 0.5 to 2,000 μm,and can be added in a desired amount to the nondrying vegetable oil. Asan oil-absorbing crosslinked polymer of this type, it is possible to use"Oleosorb PW-190" (trade name) or "Oleosorb PW-170" (trade name), anacrylate ester polymer produced by Nippon Shokubai Co., Ltd.

A description will next be made about the characteristic feature of thesecond aspect of the present invention, that is, about the rollingbearing for the high-speed rotating equipment, to which theabove-described lubricating oil according to the first aspect of thepresent invention, said lubricating oil being composed primarily of thenondrying vegetable oil, is applied.

The rolling bearing according to the present invention of the high-speedrotating equipment, to which bearing the above-described lubricating oilis applied, has a rolling bearing element for rotatably supporting atleast the rotary shaft thereon. The rolling bearing element isconstructed of the outer ring, the inner ring, the rolling elements(balls), and the retainer.

In the rolling bearing according to this invention of the high-speedrotating equipment, the construction of the retainer and the manner ofits lubrication can be summarized into the following two types:

1. (Type 1) (the construction and a first retainer and the manner of itslubrication):

(i) the retainer is made of a non-porous metal or synthetic resin, and

(ii) the retainer is lubricated with a lubricating oil composed of anondrying vegetable oil.

2. (Type 2) (the construction and a second retainer and the manner ofits lubrication):

(i) the retainer is composed of a molded porous synthetic resin bodyhaving, at least in a part thereof, a porous portion of a communicatedstructure, and

(ii) the retainer is lubricated with a lubricating oil comprising anoil-absorbing agent, which is composed of oil-absorbing synthetic resinparticles, and a nondrying vegetable oil.

In Type 1 of the present invention, the lubricating oil may containoil-absorbing synthetic resin particles like Type 2.

In the present invention, the construction of the retainer of Type 1 andthe manner of its lubrication are similar to the conventional art exceptthat the construction of the lubricating oil is substantially differentand can also be easily understood from the below-recited description ofthe retainer belonging to Type 2. A detailed description of the retainerof Type 1 is therefore omitted herein.

Concerning the rolling bearing according to the present invention of thehigh-speed rotating equipment, the construction of a retainer belongingto Type 2 will hereinafter be described in detail.

In the rolling bearing according to the present invention of thehigh-speed rotating equipment, the retainer for the rolling elements(balls), said retainer being an important element of the bearing, iscomposed of a porous body made of a particular matrix material.

A first embodiment of the retainer usable in the present invention willbe described next.

The first embodiment of the retainer usable in the present invention iscomposed of a molded body of a porous polyimide resin.

FIG. 3 is a perspective view of a retainer 44 composed of theabove-mentioned molded body of the porous polyimide resin. In thefigure, symbol 44a indicates a retainer main part, and symbol 44bdesignates pores.

Incidentally, the retainer 44 is applied to the dental air turbine handpiece of the ball bearing type described with reference to FIG. 1 toFIG. 2.

In the retainer which is composed of the molded porous polyimide resinbody and is usable in the present invention, the above-mentionedpolyimide resin (hereinafter abbreviated as "the PI resin") is a resinobtained by subjecting an aromatic carboxylic acid and an aromatic amineto condensation polymerization and containing imide bonds in thebackbone thereof (which may be either thermoplastic or thermosetting),and is excellent in heat resistance, chemical resistance, mechanicalproperties and electrical characteristics.

In the present invention, the term "PI resin" should be interpreted toalso include polyamideimide resins (hereinafter abbreviated as "the PAIresins") containing imide bonds and amide bonds in their backbones.

In the present invention, a commercially-available PI resin or PAI resincan be conveniently used as the PI resin or PAI resin for molding theretainer. As commercially-available PI resins and PAI resins, thefollowing resins can be exemplified including their chemical structuralformulas.

(i) PI resins

(1) "P94-HT" [trade name; product of Lenzing AG, Austria; represented bythe below-described chemical formula (1) in which R represents analkylene group];

(2) "TI-3000" [trade name; product of Toray Industries, Inc.;represented by the below-described chemical formula (2)];

(3) "UIP-S" [trade name; product of Ube Industries, Ltd.; represented bythe below-described chemical formula (3)];

(4) "Vespel" [trade name; product of E.I. du Pont de Nemours & Co.,Ltd.; represented by the below-described chemical formula (2)];

(5) "Aurum" [trade name; product of Mitsui-Toatsu Chemicals Inc.;represented by the below-described chemical formula (4)]; and

(6) Others, including "Meldin 8100" and "Meldin 900", products of Furon,U.S.A.

(ii) PAI resins

(1) "Torlon 4000 TF" [trade name; product of Amoco Chemical Corp.;represented by the below-described chemical formula (5) in which Phrepresents a phenylene group]. ##STR1##

The retainer composed of the above-described molded porous PAI resinbody can be produced by compressing and sintering PAI resin powder theaverage particle size of which has been classified and sifted to 15 to50 μm.

By adjusting the average particle size, pressure and the like of theresin powder as desired in the above-described sinter molding, theretainer is produced in a porous structure containing 5 to 20% by volumeof communicating pores. As will be described in detail subsequentlyherein, the communicating pores are next impregnated with a specificlubricating oil (base oil: nondrying vegetable oil) to produce theretainer according to the present invention.

If powder of fine particle sizes smaller than 15 μm is mixed in theresin powder as the raw material, the pores, namely, the spaces arefilled with such fine powder in the retainer composed of the moldedporous polyamideimide (PAI) body. This leads to variations in theporosity values of products. The above-described classification andsifting are therefore needed.

If powder of large particles sizes greater than 50 μm is mixedconversely, large spaces are formed between particles so that thepercent retention (percent oil retention) of the impregnated oil duringhigh-speed rotation is lowered. Mixing of powder of such a largeparticle size is therefore not preferred.

FIG. 4 to FIG. 5 illustrate another embodiment of the retainer 44composed of the molded porous PAI resin body.

Specifically, FIG. 4 illustrates a dental air turbine hand piece of astructure different from the above-described hand piece shown as anapplication example of a retainer 44 in FIG. 2. A description of thestructure of the dental air turbine hand piece in FIG. 4 is omittedherein, because it is clear from the above-described corresponding FIG.2.

On the other hand, FIG. 5 illustrates the shape and structure of theretainer 44 and corresponds to the above-described FIG. 3. In FIG. 5,symbol 44a indicates a main retainer part while symbol 44b designatespores.

The second embodiment of the retainer according to the present inventionwill be described next.

The second embodiment of the retainer according to the present inventionis composed of a molded porous PI resin body.

The retainer composed of the molded porous PI resin body can be producedin a similar manner as the above-described sinter molding of the powderof the PAI resin. Namely, a molded porous PI resin body containingcommunication pores in a proportion as much as 5 to 20% by volume can beproduced by compressing and sintering PI resin powder the averageparticle size of which has been classified and sifted to 15 to 50 μm.

Compared with the above-described retainer made of the porous PAI resin,the above-described retainer made of the porous PI resin has merits suchthat, when the retainer runs out of the lubricating oil, the retainerdoes not melt and the bearing does not become no longer usable and thatthe PI resin is slow in moisture-absorbing speed and hence permits easycontrol of raw material powder and molded bodies. For the reasonsmentioned above, the latter resin, i.e., the PAI resin is classified asa fusible resin.

In sterilization treatment (autoclaving), on the other hand, preventionfrom infection, especially to HIV is also required in recent years. Itis therefore required to perform the treatment under severer conditions(for example, 2.4 kg/cm², 135° C., 5 minutes) than the conventionaltreatment conditions. In spite of such severe conditions, theabove-described retainer made of the porous PI resin can withstandtemperatures up to about 200° C.

A third embodiment of the retainer according to the present inventionwill be described next.

The third embodiment of the retainer according to the present inventionis composed of a shaped phenol resin body having a fiber layer which canbe considered to be a communicated pore layer. A shaped phenol resinbody provided with a fiber layer of this type can be impregnated with alubricating oil by making use of the fiber layer.

In the present invention, the shaped phenol resin body with the resinlayer incorporated therein (which may hereinafter be called the "porousPR") has function to be impregnated with a lubricating oil through thefiber layer (fabric) although this function is different from theimpregnable function of the above-described molded porous PI or PAIresin body (which may hereinafter be called the "porous PI/PAI-R") thatthe lubricating oil is impregnated through the communication pores. Theabove-described shaped phenol resin body with the fiber layer (fabric)incorporated therein is therefore taken as a molded resin body having aporous portion (communication pores) in the present invention, althoughthe term may not be fully accurate.

The phenol resin retainer provided with a fiber layer of theabove-mentioned type can be produced, for example, by the processdisclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI6-165790.

For example, the above-described shaped phenol resin body with thefabric incorporated therein can be produced by:

(i) a process in which the fabric is rolled plural times into apipe-like shape, the rolled fabric is impregnated with a phenol resinwhile maintaining it in a vacuum state, and the phenol resin is thenhardened to form a porous cylindrical body, or

(ii) a process in which the fabric impregnated with a phenol resin isrolled plural times, and the rolled fabric is then heated under pressureinto a porous cylindrical body.

The porosity of the above-described shaped phenol resin body with thefabric incorporated therein can be similar to that of theabove-described PI resin, and a porous one having a porosity of from 5to 20% by volume can be used.

The present invention will hereinafter be described in further detail bythe following example.

In particular, lubricating oils according to the present invention, eachof which was composed primarily of a nondrying vegetable oil and isapplicable to a rolling bearing in a dental air hand piece, werecompared with various known lubricating oils to substantiate thesuperiority of the lubricating oils of the present invention.

The bearing unit of a dental air hand piece (see FIG. 1 to FIG. 2)furnished for use in a test of this example had the followingconstruction:

Miniature rolling bearing of the open type, which was provided with asnap retainer and had the following dimensions:

(i) Inner diameter of an outer ring: 6.350 mm

(ii) Inner diameter of an inner ring: 3.175 mm

(iii) Width: 2.380 mm

The above bearing unit was assembled in the dental air turbine handpiece, and the test was conducted under the following conditions: airsupply pressure, 2.5 kg/cm² ; air supply rate, 26 l/min; revolutionspeed, about 400,000 rpm.

Namely, using the dental air turbine hand piece (see FIG. 1 to FIG. 2),the lubricating oils according to the present invention and the variousknown lubricating oils were tested under the above-described conditions.The test results are presented in Table 3.

In Table 3, the notes have the following meanings:

(1) Heat resistance test (autoclaving resistance; cycles)

An autoclaving apparatus ("ALPHI", trade name; manufactured by J. MORITAMFG. CORP.) was used. Autoclaving resistance is expressed in terms ofcycles until the rotation of the dental air turbine hand piece becameunstable and the efficiency of rotation dropped to 10% (about 40,000rpm).

Conditions for the treatment in the autoclaving apparatus were: steampressure, 2.4 kg/cm² ; temperature, 135° C.; time, 5 minutes.

(2) Bearing resistance (continuous operation; hrs)

Each lubricating oil was applied to a lubrication system of the dentalair hand piece at the beginning, and a continuous operation was thenperformed at about 400,000 rpm without additional feeding of thelubricating oil. The bearing resistance is expressed in term of hoursuntil the rotation become unstable and the revolution speed dropped to10% (about 40,000 rpm).

The specific constructions of various retainers shown in Table 3 are asfollows:

(i) Non-porous PI/PAI.R

This means a non-porous retainer of the polyimide (PI) base or thepolyamideimide (PAI) base.

The non-porous PI.R was obtained by forming "Vespel SP-1" (trade name,product of E.I. du Pont de Nemours & Co., Ltd.) into a retainer-likeshape.

On the other hand, the non-porous PAI.R was obtained by forming "Torlon4203" (trade name, product of Teijin-Amoco Engineering Plastics, Ltd.)into a retainer-like shape.

(ii) Porous P.R

This means a retainer of the phenol resin base. Incidentally, the porousP.R was obtained by impregnating inter-thread spaces of a woven fabric,which had been rolled plural times into a pipe-like shape, with a phenolresin in a vacuum state, shaping the thus-impregnated fabric under heatand then machining the shaped fabric into a retainer-like shape.

(iii) Porous PI/PAI.R

This means a porous retainer composed of a sintered body of polyimide(PI) base or polyamideimide (PAI) base powder.

The porous PI.R was obtained by compression-forming "UIP-S" (trade name,product of Ube Industries, Ltd.) under a forming pressure of 4000kgf/cm², sintering the green compact at 400° C. in a nitrogen gasatmosphere and then machining the sintered compact into a retainer-likeshape (porosity: about 13% by volume).

The porous PAI.R was obtained by classifying and sifting "Torlon 4000TF"(trade name, product of Amoco Chemical Corp., U.S.A.) into an averageparticle size of 20 μm, compression-forming the thus-shifted powderunder a preforming pressure of 2,800 kgf/cm², sintering the greencompact at 300° C., and then machining the sintered compact into aretainer-like shape (porosity: about 14% by volume).

In Table 3,

(a) As the paraffin oil (liquid paraffin), a conventional spray-typeparaffin oil produced by an odontotherapy-related maker was used.

(b) As the fluorinated oil, "FOMBLIN" (trade name, product of AusimontS.p.A., Italy) was used.

(c) As the oil-absorbing crosslinked polymer of the acrylate ester base,"PW-170" (trade name, product of Nippon Shokubai Co., Ltd.) was used.

                                      TABLE 3                                     __________________________________________________________________________    Evaluation of Characteristics of Various Lubricating Oils                                              Heat resistance (1)                                                                          Bearing resistance (2)                                         (autoclaving resistance, cycles)                                                             (continuous rotation, hrs)                                     (i)   (ii)                                                                              (iii)                                                                              (i)   (ii)                                                                              (iii)                                         Environmental                                                                        Non-porous                                                                          Porous                                                                            Porous                                                                             Non-porous                                                                          Porous                                                                            Porous                      Lubricating oil                                                                            Biosafety                                                                          Conservation                                                                         PI/PAI · R                                                                 P · R                                                                    PI/PAI · R                                                                PI/PAI · R                                                                 P · R                                                                    PI/PAI ·           __________________________________________________________________________                                                      R                           Vegetable oil                                                                 Nondrying oil                                                                 Refined olive oil                                                                          A    B      1000< --  --   1.50  --  --                          Extra virgin olive oil                                                                     A    B      1000< --  --   2.00  --  --                          Virgin olive oil                                                                           A    B      1000< --  --   1.75  --  --                          Arachis oil  A    B      1000< --  --   1.50  --  --                          Extra virgin olive oil                                                                     A    B      1000< 1000<                                                                             1000<                                                                              3.50  70.75                                                                             119.25                      100 parts by weight +                                                         oil-absorbing polymer                                                         of acrylate ester base                                                        5 parts by weight                                                             Semidrying oil                                                                Rapeseed oil A    B      300   300 300  0.25  2.25                                                                              4.50                        Mineral oil                                                                   Paraffin oil C    C      1000< 200 1000<                                                                              0.50  6.75                                                                              8.50                        Paraffin 100 parts by weight +                                                             C    C      1000< 150 1000<                                                                              1.00  4.75                                                                              17.00                       acrylic resin 5 parts by weight                                               Synthetic oil                                                                 Fluorinated oil                                                                            C    C      1000< 1000<                                                                             1000<                                                                              1.50  14.50                                                                             17.50                       __________________________________________________________________________     A: Excellent, B: Good, C: Poor                                           

As is shown in Table 3, it is appreciated that the lubricating oilsaccording to the present invention, said lubricating oils being composedprimarily of the nondrying vegetable oil respectively, are excellent inthe various characteristics of biosafety, environmental conservation,heat resistance (autoclaving resistance) and bearing durability comparedwith the conventional lubricating oils. In particular, it is understoodthat the lubricating oil prepared by adding the oil-absorbing polymer ofthe acrylate ester base to the nondrying vegetable oil has been markedlyimproved in these various characteristics.

We claim:
 1. A lubricating oil for a rolling bearing in medicalhigh-speed rotating equipment, wherein said lubricating oil comprises anon-drying vegetable oil, and said non-drying vegetable oil comprises:atleast 60 wt. % of at least one mono-unsaturated fatty acid, saidmono-unsaturated fatty acid containing one unsaturated bond permolecule; and at most 30 wt. % of at least one polyunsaturated fattyacid, said poly-unsaturated fatty acid containing at least two saturatedbonds per molecule; and wherein said non-drying vegetable oil has a lowfree acid value.
 2. The lubricating oil according to claim 1, whereinsaid nondrying vegetable oil comprises at most 10 wt. % of a tocopherol.3. The lubricating oil according to claim 1, wherein said nondryingvegetable oil is selected from a group consisting of olive oil, arachisoil and oleysol oil.
 4. The lubricating oil according to claim 1,wherein said nondrying vegetable oil comprises at most 5 wt. % ofsaturated and unsaturated fatty acids not combined with glycerin (freefatty acids).
 5. A lubricating oil for a rolling bearing in medicalhigh-speed rotating equipment according to claim 1, wherein saidnon-drying vegetable oil has a low free acid value of 0.1 wt. %.
 6. Alubricating oil for a rolling bearing in medical high-speed rotatingequipment, wherein said lubricating oil comprises a non-drying vegetableoil, and said non-drying vegetable oil comprises:at least 60 wt. % of atleast one mono-unsaturated fatty acid, said mono-unsaturated fatty acidcontaining one unsaturated bond per molecule; and at most 30 wt. % of atleast one poly-unsaturated fatty acid, said poly-unsaturated fatty acidcontaining at least two unsaturated bonds per molecule; and wherein saidlubricating oil is made by adding 5 to 10% aqueous solution of sodiumhydroxide to the non-drying vegetable oil, heating the non-dryingvegetable oil with the sodium hydroxide to saponify the non-dryingvegetable oil and centrifuging said saponified non-drying vegetable oilto provide a low free acid non-drying vegetable oil.